– Pushpa Raj Adhikary

We call the pull of the earth on the bodies the force of gravity. The measure of this pull is called the weight of the body. There is no escape from the gravity and its eternal laws are valid even in the remotest parts of the universe. It equally pervades vacuum and the densest substance. There is no way of shielding from it or acting on it. Its action is less and less when we move away from earth but does not vanish completely. Gravity makes rivers flow down to the sea, keeps the atmosphere around the earth, and is the cause of tides in the oceans. We have to use force to overcome gravity if we want to move away from the earth.

Since time immemorial, living beings had to reckon with gravity, and learned to adapt to it. The force of gravity, which makes everything move towards it, was unexplained for ages. The first man to develop a scientific theory of gravity and apply it to study of the universe was the great Englishman, Sir Isaac Newton.

The anecdote that Newton discovered the law of gravity by watching an apple fall from a tree may or may not be true. It has been said that he invented this story to get rid of people demanding explanation of just how he discovered the great law. Today, any high school student knows this law with such an ease that it seems strange indeed that there was a time when learned men had not the slightest idea about it. However, it is not as it may appear to us and it took the genius of Newton to discover it.

Newton’s studies convinced him that not only earth attracts an apple but an apple also attracts the earth. In fact, every material body attracts other material bodies towards it. But then why the apple moves towards the earth but not the earth towards the apple? This attraction or pull or force exists between the earth and all heavenly bodies too. This is known as the force of gravitation.Any material object attracts all other material objects and this attraction is in proportion to the weight of an object. The heavier a body, the stronger is the attraction. The weight of the earth is enormous compared to the weight of an apple or a man. Hence, the attraction exerted by the earth on other objects is also very strong compared to the attraction of an apple on earth or by a man on earth. This attraction of the earth makes every body move towards earth. The attraction between two material bodies increases if they come closer or if their weights are increased.

About seventy years before Newton’s time, the great German Scientist Johannes Kepler discovered the law as how planets moved around the sun. But in Kepler’s time nobody knew why the planets moved as explained by him. Newton, with the help of the law of universal gravitation, could explain why the planets moved around the sun as explained by Kepler. The universal law of gravitation found another brilliant confirmation in the discovery of the planet Neptune. Astronomers had long discovered that the planet Uranus occasionally appeared to stray from its orbit. Sometimes it would slow down its motion and again it would go faster as if drawn by some invisible force. The law of gravitation predicted that the anomaly in the motion of Uranus was due to the presence of another planet farther from Uranus and soon astronomers discovered a new planet Neptune.

For many decades Newton’s theory of Gravitation appeared perfect. But then facts began to accumulate which could not be explained by the law of universal gravitation alone. One of these is the Seeliger paradox. This paradox goes this way. The universe is infinite and is infinitely variable. Its lifetime too, is unlimited. It is more or less filled with material bodies and so can be assumed to possess some mean density of matter. Seelinger decided to apply the universal law of gravitation to determine the gravitational force which an infinite universe would exert at any point within it. This force was found proportional to the radius of the universe. As the radius of the universe is infinite, so the force would be. But this is not the case. Does it mean that the law of Universal gravitation is not valid on universal scale?

Another phenomenon in which the conclusions of gravitational theory did not quite agree with observations was found in the displacement of the orbit if the planet Mercury. Very accurate calculations of the orbit of Mercury reveal that the point closer to the sun suffers a precession or displacement. For a long time this precession of the orbits of Mercury remained unexplained. It took a revolution in science to explain it, and the revolution was carried out by a young German Scientist, Albert Einstein.

It is a long known fact that if a gun fires at a distance we see the flash of light some time before we hear the sound. This tells us that sound travels in a far less speed that the light. It was possible to measure the speed of sound in the surface of the earth as 330 meters per second. But it is much harder to measure the speed of light because light travels with an incredible speed of 3,00000 kilometers per second. A ray of light can circle the earth in just over 0.1 second i.e. one tenth of a second. For a long time people were unable to measure the speed of light. It was finally measured by observing the eclipses of the satellites of the planet Jupiter from two points on earth’s orbit around the sun, when the earth was closed and farther from Jupiter. Today it is measured in laboratory conditions to a high degree of precession by means of rotating mirrors. In fact, not only light but all electromagnetic waves travel with light’s speed as the electromagnetic field moves through space.

But how do electromagnetic fields propagate through space? Does gravitational force also propagate through space in the form of gravitational field? If so, how fast does a gravitational field travel? As fast as sound in air, light in vacuum or with some other speed? Can the attraction between the bodies happen directly without the participation of the intervening medium? Do the gravitational force and gravitational field also propagate with the same speed of light or have a finite velocity? A new scientific theory was needed to explain the propagation of electromagnetic field through space and its foundation was laid in 1905-1915 by Albert Einstein in his special and general theories of relativity based on the geometries of Lobachevski and Riemann.

One of the fundamental conclusions of the special theory of relativity, which defines the interconnection between space and time, is the equivalence of mass and energy. The theory states that a moving body carries kinetic energy, hence its mass is greater than when it is at rest. The greater a body’s latent energy is, the greater is its mass. A cup of hot coffee is heavier than cold coffee in the same cup. The famous equation E=mc² is Einstein’s formula of mass-energy equivalence.

But what is meant by a body’s mass? The mechanical concept of mass states that mass is a measure of a body’s inertia. Hence, mass can be expressed in terms of force and the acceleration which it imparts to the body. In physics, mass measured in this way is known as inertial mass. But mass can also be measured from Newton’s formula of gravitation. This mass of bodies which may be at rest relative to one another is known as gravitational mass. The physical interpretation of inertial and gravitational mass are different but quantitatively have, to date, been found to be the same no matter how they are measured. This led Einstein to think that inertia and gravitation must have a common origin. So, if a body’s inertial mass varies with the velocity of motion, then, he reasoned, the gravitational mass should also vary with the velocity of motion.

Einstein’s identification of inertia and gravity on the basis of the equality of inertial and gravitational mass of great significance. It enabled him, in 1915, to develop the general theory of relativity, which is the modern theory of gravitation. This modern theory offers a much more exact and profound explanation of the properties of the bodies than Newton’s theory. Einstein’s theory was a revolution in physics which provided explanation for many hitherto unexplained phenomena. But it would hardly be useful to present the theory in common language as it contains largely mathematical, extremely complicated equations belonging to the class of non-linear differential equations in spite of the clarity of its physical meaning.